This invention relates to the cutting of glass, using a cold fluid jet to produce a controlled thermal shock to induce a precisely propagated fracture of the glass. While not limited thereto in its capability, the present invention is an effective tool for severing glass and similar material in curved patterns and odd shapes as well as linear cuts without leaving serrated, or ragged edges.
In U.S. Pat. No. 1,720,883 F. N. Campbell, et al disclose a method of severing glass along a defined line by applying heat to the zone of such line and other conditions in a manner whereby controlled expansion takes place and the glass is accurately severed at the desired line. The heat necessary for the invention is provided in a controlled method from a table and support system which separates the glass and pieces derived from the cutting process. The prior art is limited in its flexibility; that is, the pattern in which the glass is to be cut must be pre-determined and then the mechanism, or table must be reset each time a change in the cutting pattern is desired. The prior art utilizes bare copper wire laid within notches on the cutting table to provide heat for the cutting process, therefore, this method is further inflexible because its mode of thermal transference must also be prepatterned.
In U.S. Pat. No. 3,839,006 Pikor discloses an apparatus for cutting notch sensitive materials. The apparatus induces heat shock to notch sensitive materials, including glass. This apparatus includes a flexible resistance heater wire which may be applied to an article to be severed in overlying relationship to the desired fracture plane; the fracture plane usually being defined by a preconditioning step of scribing. The prior art is inflexible in its ability to transfer the necessary heat for the process because the mode of tranference is a copper wire, patterns of severence are limited to shapes which can be overlayed by the wire. Pikor's is also inflexible in that the glass must be scribed along the desired cut before the mode of heat transfer is applied to sever the glass.
In United Kingdom patent number 1,357,116 Pilkington Brothers Limited disclose an invention which relates to the cutting of glass and in particular to the cutting of glass sheet material by inducing thermal stress within the glass. The apparatus provides a method of cutting glass sheet material which method comprises forming a point of weakness at a position on the glass, directing a stream of heated gas through a nozzle outlet towards the glass so that the stream is incident on the point of weakness to initiate a fracture in the glass, meanwhile restraining the flow of heated gas away from the area of glass adjacent the nozzle outlet by means of a restraining member symmetrically disposed around the nozzle outlet and extending perpendicular to the axis of the nozzle outlet, and effecting relative movement between the glass and the nozzle outlet along a path, the temperature and flow rate of the gas being controlled so that sufficient stress is generated in the glass to extend the fracture from the point of weakness along the path in a controlled fashion.
In summary, prior art claims to generate a fracture by heating the glass by direct contact with a heated wire, heating of the glass by bringing the glass in close proximity to a heating wire, or, by heating the glass with a jet of hot gas. While heating the glass by any of the above methods will result in a fracture of the glass, precise control of the fracture line is difficult to achieve because the fracture actually occurs as the glass cools. Thus, the prior art relies on precision heating of the glass along the desired fracture line and depends upon ambient atmospheric air to cool the glass back to the the line of fracture. At best, this is a random cooling process, particularly where the heating and cooling is not in a linear pattern. The prior art is limited, as cited in United Kingdom patent number 1,357,116, Pilkington Brothers Limited, in that small circles are impossible to cut in their entirety by the hot gas jet method of heating. The final few degrees cannot be cut by the hot gas method if the ambient atmospheric air is used for cooling, because the final few degrees will fracture uncontrollably in an undesireable fashion in a chord to the circle, therefore, use of mechanical means is required to complete the cut. This undesireable effect is caused by the retention of heat in the glass in the center of circle which is subjected to constant application of heat; the retained heat cannot be disipated quickly enough by ambient atmospheric air. However, if the exact track of the cooling gradient can be controlled, a precise, and controlled fracture can be achieved. The prior art attempts to control the fracture line by surrounding the hot air jet with a ring to control lateral dispersion of the hot air and consequently disipation of heat through the glass. The exact line of fracture is dependant upon the rate of cooling from the outside of the heated area to center of the area which was heated. The present invention directs the cooling air to cause a fracture in any desired pattern and provides a finished edge that requires no additional grinding, or cutting.